Glass materials have been used in various applications, and surface polishing is required depending on the application thereof in some cases. For example, mirror finish is required for optical lenses. Also, for use in glass substrates for optical disks and magnetic disks, glass substrates for thin film transistor (TFT) type or super-twisted nematic (STN) type liquid crystal displays (LCD), color filters for a liquid crystal television, and glass substrates for photomasks for a large scale integrated circuit (LSI), more precise surface polishing is required to achieve high flatness with a small surface roughness, and, in addition, to have no defects.
Cerium-based abrasives have been conventionally used in surface polishing of such glass materials. The cerium-based abrasives typically contain about several % fluorine atoms so as to increase the polishing power.
Although the detailed polishing mechanism of cerium-based abrasives is not clear, it has been confirmed that polishing is accelerated by the combined functions of a mechanical effect due to the hardness of the cerium oxide particles themselves, and a chemical effect due to the contained fluorine. It is also known that in cases where fluorine is contained in advance in raw materials to be fired when manufacturing abrasives, sintering of the constituent particles of the abrasive when firing is appropriately accelerated to obtain abrasives exhibiting high polishing power.
The surface precision required for glass substrates, however, has become more severe in recent years, and cerium-based abrasives are also required not to cause surface defects such as scratches, pits, and the deposition of abrasive particles, even at a high polishing rate.
Corresponding to the requirements, for example, PTL1 proposes the improvement in the polishing power and the polishing precision by reducing the content of fluorine atoms in view of environmental concerns, and controlling the alkali metal content at 0.01 to 3.0 mass % and the chlorine concentration at 0.3 mass % or less relative to the total rare earth elements in terms of oxides (TREO).
Further, PTL2 describes a 2-component cerium-based abrasive which contains fluorine atoms and rare earth elements substantially composed of cerium and lanthanum through control of the content of oxides of rare earth elements other than cerium such as neodymium and praseodymium.